The present invention relates to improvements in control elements for gearbox ratio selectors in heavy vehicles.
Vehicles are generally provided, between the engine and the driving wheels, with a gearbox which permits the driver to choose, from a determined number of possibilities, the most suitable to the road conditions on which he travels.
This solution, which is perfectly valid for all types of vehicles, is frequently not so when the vehicle must transport heavy loads.
Thus, in the case of heavy-tonnage trucks the solution consisting of a reduction set which cuts the performance of the gearbox in each one of its speeds is adopted. Due to this solution the possibilities of selection are increased, permitting any circumstance presented during transport to be confronted with, the engine not losing any power.
However, this solution, known for a long time, presents in practice technical problems which complicate driving. In fact, when changing from a normal speed ratio to another selected reduced speed ratio, two levers must be handled, one of which operates the reducer while the other operates the selector.
With this described solution, the vehicle must be stopped and the controls of the gearbox and of the reducer must be placed in neutral with the vehicle at rest, whereby the possibilities of use are limited.
The application of pneumatically operated gearboxes permits the driver to change from one speed to another, and even from one ratio, to another, without having to stop the vehicle. Thereby, depending on the needs of the load and on the road, the driver can change from one ratio to another, i.e. from high speeds to low speeds, conserving the inertia of the vehicle.
In spite of this improvement, the driver still has to handle two levers as simultaneously as possible in order to obtain the sought after effects, making a rapid movement in which he has to use both hands, thereby releasing the steering wheel.
A known solution to this problem is that the controls of both the gearbox and the reduction gear are included in a single lever, so that the driver can simultaneously and with a single hand select and operate any one of the speeds, at a high or normal ratio or a reduced ratio, using his free hand to firmly grip the steering wheel.
According to such solution, there is provided in the handle of the gearshift lever a compressed air supply distributor receiving compressed air from a supply and which, through conduits, operates a double cylinder which permits changes from low to high speeds and vice versa, without the need of an additional control lever.
The distributor placed in the interior of the handle of the operating lever of the normal gearshift comprises an inner chamber wherein there is housed a rotary distributor provided with an axial hole through which passes compressed air from a compressor. This chamber has communication with the compressor, at the outlet of which others are arranged about a housing in which the rotary distributor is introduced.
To the outlet of the conduits of the fixed housing in which the rotary distributor is introduced, there are connected the conduits which, passing through the interior of the lever arm of the gearshift mechanism, lead to the feed inlets and outlets of each one of the operating cylinders of the gearshift fork.
Depending on the angular position of the handle of the gearshift mechanism, with relation to the arm of the lever of the standard mechanism, the compressed air of the compressor is distributed, thus activating the pneumatic cylinders in one direction or the other.
This solution, already known, has given satisfactory results in practice and has been adopted by various manufacturers specialized in heavy vehicles. However, a delay effect between the activation of the handle and the reply from the operating cylinder of the reduction gearing has been observed.
This delay can, under certain circumstances, prevent changes from a normal speed ratio to a reduced speed ratio due to the fact that the peripheral speeds of the pinions of the gearbox increase with the inertia of the vehicle, impeding the action of the selector and throwing out the gear, with the consequent problem.
Also, the loss in speed of the vehicle during the change can be such that the low-speed engine will not have sufficient power to haul the vehicle and to recover the revolutions.
In any case, these situations are caused by the delay effect produced due to the small size of the diameter of the conduits which distribute and direct the compressed air through the handle and the gearshift lever from the supply of compressed air to the operating cylinders.